Digital radio communication systems are known in the art. Such systems typically include a plurality of audio source units, a plurality of audio destination units, a switching unit for selecting which of the plurality of audio source units is presently operable, and a plurality of communication units. Switching from one audio source unit to another (e.g., as a result of a mobile communication unit roaming from one coverage area to another) causes unintelligible audio at the receiving end. A typical digital radio communication system employs encryption parameters at the sourcing end to provide encrypted voice to the receiving communication unit. Thus, the receiving communication unit must have the corresponding decrypting parameters to properly decode the received encrypted signals, as next described.
Upon reception of a preamble signal by the communication unit, the communication unit examines a so-called encryption synchronization (ESYNC) field of the preamble, and adjusts the encryption algorithm and secure key for the duration of the call. If the audio source changes, the communication unit must reset its operating parameters (e.g., encryption and secure key parameters) to ensure compatibility with the new audio source unit. However, if the communication unit is not notified that there is a new audio source unit, the communication unit attempts to decrypt the new audio using the old parameters, thereby providing unrecognizable audio to the user.
One technique for notifying the receiving communication unit that the audio source unit has changed is to encode each audio packet with control information identifying the audio source for that packet. However, this approach requires an undesirable amount of bandwidth, which could otherwise be used to convey speech.
Another technique for notifying the receiving radio that the audio source unit has changed is to have the radio automatically look for a new encryption parameter upon radio detection of a switch (e.g., garbled audio followed by a mute). However, this would likely result in undesirable audio delays and perhaps even lost speech.
Accordingly, there exists a need for a method of facilitating an audio source change as between a plurality of audio source units. In particular, a method is needed that automatically determines the occurrence of an audio source change based on known expected frame sequences, thereby resulting in facilitation of the audio source change without the constraints of prior art systems.